This application claims the benefit of Japanese Patent Application No. 08-25504 filed Sep. 4, 1996.
1. Field of the Invention
The present invention concerns a transmission illumination type differential interference microscope.
2. Discussion of the Related Art
One example of the construction of a conventional transmission illumination type differential interference microscope is shown in FIG. 6.
Illuminating light from a light source 10 is focused by a collector lens 11, and then illuminates a sample 15 on a slide glass 14 via a condenser lens 13. Light from the illuminated sample 15 is focused by an objective 16, so that an enlarged image 18 is formed. The observer observes this enlarged image 18 with eye 20 via an ocular lens 19. A polarizer 12 and a first Wollaston prism 5 are installed, in that order, in the light path between the collector lens 11 and the condenser lens 13; furthermore, a second Wollaston prism 6 and an analyzer 17 are installed, in that order, in the light path between the objective 16 and the enlarged image 18. The first Wollaston prism 5 is positioned at a front-side focal plane of the condenser lens 13, i.e., at the light-source side focal plane of the condenser lens 13, while the second Wollaston prism is positioned at the rear-side focal plane of the objective 16, i.e., at the image-side focal plane of the objective 16.
The principle of differential interference microscope, in the above-mentioned construction, will be outlined with reference to FIG. 6. Illuminating light from the light source 10, which has been focused via the collector lens 11, is converted into linearly polarized light whose plane of vibration is inclined 45xc2x0 with respect to the plane of the page by the polarizer 12. As a result of the birefringent action of the first Wollaston prism 5, the light from the polarizer 12 is separated into linearly polarized light L1, which vibrates in the direction perpendicular to the plane of the page, and linearly polarized light L2, which vibrates in a direction parallel to the plane of the page, i.e., into two light beams which are perpendicular to the optical-axis and perpendicular to each other. Both light beams L1 and L2 proceed with a small angle of separation after passing through the first Wollaston prism 5, and reach the sample 15 as substantially parallel light beams as a result of the focusing action of the condenser lens 13.
After both light beams L1 and L2 pass through separate positions on the sample 15, the light beams are focused on the second Wollaston prism 6 by the focusing action of the objective 16, and these light beams are again induced to proceed along the same light path by the birefringent action of the second Wollaston prism 6. Of the two substantially perpendicular linearly polarized light components of the light beams L1 and L2, only the vibrational component, in the direction of the transmission axis of the analyzer 17, is extracted by the analyzer 17 and induced to interfere, and the interference fringes, corresponding to the phase difference applied to the two light beams L1 and L2 inside the sample, are observed.
The constructions of the first and second Wollaston prisms 5 and 6 used as birefringent members, in the above-mentioned conventional example will now be described with reference to FIG. 7. In a Wollaston prism, such as the prism 5, a pair of wedge-shaped prisms 5a and 5b consisting of an optical material, which possesses birefringence, e.g., a crystal such as quartz crystal, calcite, or sapphire, etc., are connected together so that the optic-axis 5bx of wedge-shaped prism 5b is parallel to the joining surface 5s and perpendicular to the optical-axis Z, and so that the optic-axis 5ax of wedge-shaped prism 5a is perpendicular to both the optic-axis 5bx of the first prism 5b and the optical-axis Z.
In FIG. 7, the optic-axis 5ax of the wedge-shaped prism 5a, positioned on the side from which the light beams are incident on the Wollaston prism, is oriented parallel to the plane of the page (as indicated by the arrows in the figure), while the optic-axis 5bx of the wedge-shaped prism 5b, which is also denoted as emission-side prism, is perpendicularly oriented to the plane of the page (as indicated by the + sign in FIG. 7), and both optic-axes 5ax and 5bx are oriented so that the directions of these optic-axes are perpendicular to the optical-axis Z. However, this combination of optic-axis orientations could also be arranged so that the orientations on an incident side, wedge-shaped prism 5a, and the emission side are reversed, i.e., so that optic-axis 5ax of the wedge-shaped prism 5a, which is also denoted as the incident-side prism, is perpendicular to the plane of the page, and so that the optic-axis 5bx of the emission-side prism 5b is parallel to the plane of the page.
The above is an example of the construction of a birefringent optical member in a case where the respective focal planes of the condenser lens 13 and objective 16 lie outside the respective lenses 13 and 16. However, in cases where lenses 13 and 16 are each constructed of a plurality of lenses, the focal planes are capable of being positioned inside the respective lenses. In this type of configuration, the Wollaston prisms 5 and 6 cannot be positioned at the focal planes of lens 13 and 16, because the respective focal planes lie within the lenses, therefore, a Nomarski prism 7, which is modified Wollaston prism such as that depicted in FIG. 8, may be utilized.
The Nomarski prism 7 is a prism in which the optic-axis 7bx of prism 7b is oriented parallel to the joining surface 7s and perpendicular to the optical-axis Z, while the optic-axis 7ax of the prism 7a is oriented perpendicular to the optic-axis 7bx of the prism 7b and inclined by an angle xcex8 from a plane that is perpendicular to the optical-axis Z. By using such a construction, it is possible to position the separation point SP of the two light beams (which was located inside the prism in the case of the Wollaston prism shown in FIG. 7) outside the prism. By positioning the separation point SP of the two beams located outside the prism at the focal planes of the respective lenses 13 and 16, it is possible to obtain an effect similar to that obtained when Wollaston prisms are positioned at these focal planes.
The Wollaston prisms 5 and 6 or the Nomarski prism 7 used as birefringent optical members, in the above-described conventional examples, are prisms in which two wedge-shaped prisms, 5a and 5b or 7a and 7b consisting of an optical material such as quartz crystal, calcite, or sapphire, etc., which possesses birefringence, are joined together so that the orientations of the respective optic axes 5ax and 5bx or 7ax and 7bx differ, as shown in FIG. 7 or FIG. 8. However, the above-mentioned birefringent materials are generally expensive; accordingly, it is difficult to lower the cost of differential interference microscopes which require such members.
Furthermore, in the birefringent materials, which make up these birefringent optical members, the refractive index to extraordinary ray (linearly polarized light which vibrates parallel to the plane determined by the optic-axis of the crystal and the normal axis of the wavefront) varies with to the above-mentioned angle of incidence. Additionally, the light path of any extraordinary rays passing through the birefringent materials also varies in accordance with the variation in the refractive index. Accordingly, in a conventional example, the phase difference between the separated light beams L1 and L2 generated when the light passes through the birefringent optical members 5 and 6 varies according to the angle of incidence of the incident light L with respect to the birefringent optical members 5 and 6.
Thus, within the field of observation of the microscope, the light which reaches the peripheral of the visual field corresponds to light which is obliquely incident on the birefringent members 5 and 6, while light which reaches the central portion of the visual field corresponds to light which is perpendicularly incident on the birefringent members 5 and 6. Therefore, within the field of observation, the phase difference in the peripheral portions of the field and the phase difference in the center of the field are different, and this difference appears as variances in the brightness and coloring within the field of observation.
The present invention is directed to a differential interference microscope, and the like, that substantially obviates one or more of the above problems due to limitations and disadvantages of the related art.
A first object of the present invention is to reduce the cost of a differential interference microscope by providing polarized light separating elements, which have a polarized light separating function comparable to that of birefringent optical members constructed entirely from conventional birefringent materials, and which contain optical materials that are less expensive than conventional materials.
A second object of the present invention is to reduce variances in the brightness and coloring in the differential interference microscope by providing polarized light separating elements in which the amount of variation in the phase difference of obliquely incident light relative to the phase difference of perpendicularly incident light that occurs when the light passes through the polarized light separating elements is smaller than in conventional polarized light separating elements.
In order to achieve the objects of the present invention, birefringent members are constructed using an isotropic material, such as glass, etc., which does not have birefringent properties in one of the prisms instead of using birefringent members consisting of a pair of prisms constructed using birefringent materials such as quartz crystal, calcite, or sapphire, etc., as in the conventional case.
Specifically, the present invention is a transmission illumination type differential interference microscope in which light in a given polarized state is separated by means of a first birefringent optical member into two linearly polarized light components whose planes of vibration are perpendicular to each other, both of the separated polarized light components are converted into parallel light components by means of a condenser lens where the object being examined is illuminated by the polarized light components which have been converted into parallel light component. Both of the polarized light components, which have passed through the object being examined are then converted into convergent light by means of an objective and both of the polarized light components, which have been converted into convergent light are synthesized into a single light beam by means of a second birefringent optical member. Both of the polarized light components of the synthesized light beam are then caused to undergo polarization interference, and an enlarged image of the object being examined is imaged by the objective using the interfering light.
The first and second birefringent optical members are each formed by joining only two wedge-shaped prisms, and in at least one of the first and second birefringent optical members, one of the two wedge-shaped prisms constituting the birefringent optical member is an isotropic prism consisting of an isotropic optical material, while the other wedge-shaped prism is a birefringent prism consisting of a birefringent optical material.
The second object of the present invention is achieved by constructing first and second birefringent members so that one of the two wedge-shaped prisms constituting each of the birefringent optical members is an isotropic prism consisting of an isotropic optical material, while the other wedge-shaped prism is a birefringent prism consisting of a birefringent optical material. The first and second birefringent optical members are positioned so that the phase difference between the separated polarized light components generated by the first birefringent member is canceled by the second birefringent member.
Thus, in the present invention, the birefringent optical members used in a differential interference microscope are respectively constructed by joining together two wedge-shaped prisms, on the condenser lens side and on the objective side. Among these birefringent optical members, at least one birefringent optical member is constructed by joining an isotropic prism consisting of an isotropic material such as glass, etc., and a birefringent prism consisting of a birefringent material such as quartz crystal, etc.
For example, in cases where both the front-side focal plane of the condenser lens and the rear-side focal plane of the objective lie outside the respective lenses, or in cases where one of these focal planes, i.e., either the front-side focal plane of the condenser lens or the rear-side focal plane of the objective, lie outside the corresponding lens, one of the two wedge-shaped prisms constituting the first birefringent member, i.e., either the incident-side prism or the emission-side prism, is formed as an isotropic prism, while the other prism is formed as a birefringent prism. Similarly, of the two wedge-shaped prisms constituting the second birefringent member, one of the prisms is formed as an isotropic prism, while the other prism is formed as a birefringent prism.
Furthermore, in cases where both the front-side focal plane of the condenser lens and the rear-side focal plane of the objective lie inside the corresponding lenses, either or both of the birefringent members are constructed by joining an isotropic prism and a birefringent prism in the same manner as described above, or only one of the total of four wedge-shaped prisms constituting the two birefringent members is formed as an isotropic prism.
Therefore, in the present invention, birefringent members are constructed by replacing either one or two of the total of four wedge-shaped prisms consisting of birefringent optical materials used in the above-mentioned prior art with an isotropic material such as glass, etc., which does not have birefringent properties. Generally, isotropic materials such as glass, etc., are less expensive than birefringent materials; accordingly, the cost of an optical system requiring birefringent optical members, and especially the cost of a transmission illumination type differential interference microscope, can be reduced.
Additionally, in the present invention, in cases where the first and second birefringent members are constructed as birefringent prisms in which one of the above-mentioned two wedge-shaped prisms constituting each of these birefringent optical members is formed as an isotropic prism consisting of an isotropic optical material, while the other wedge-shaped prism is formed as a birefringent prism consisting of a birefringent optical material, the optical path lengths of the first and second birefringent members can be shortened by approximately xc2xd compared to the optical path lengths of conventional birefringent members. Accordingly, the phase difference between the separate light beams separated by the first birefringent member can be canceled by applying a phase difference in opposition to this phase difference when the separate light beams are synthesized by the second birefringent member. In this case, the phase difference between the light that is obliquely incident on the first and second birefringent optical materials and the light that is perpendicularly incident is small, so that variances in brightness and coloring within the field of observation can be reduced.